91Ó°ÊÓ

The mass, rotation, and speed of an object, as well as the radius of the object, determine its angular momentum.

The Yo-yo is basically an energy converting machine. When you hold it, it has a potential energy. But when you release it, the potential energy gradually converted to kinetic energy, because it started to move.

Then the yo-yo is spinning at the bottom of its string, and all the potential energy it had originally has been converted into kinetic energy.

The momentary tug on the string causes the friction between the string and the axle. When the axle stops slipping, the angular momentum of the spinning yo-yo is sufficient to cause the string to wind around the axle. This causes the yo-yo to begin to 'climb' back up the string.

The angular momentum or kinetic energy of rotation of the yo-yo is turned into gravitational potential when the yo-yo climbs back up the string, corresponding to the increasing height of the yo-center yo's of mass. As the yo-yo rises, the angular momentum and, hence, the rotation rate of the yo-yo reduces. When the yo-yo was dropped, the procedure was reversed.